Introduction
Radioactive waste arising from nuclear power generation, as well as from medicine, industry and scientific research, poses a potential hazard over extended periods of time and must therefore be isolated from the human environment. The disposal of such wastes in engineered caverns far below the Earth's surface fulfills this purpose. The Laboratory for Waste Management (LES), in co-operation with the National Cooperative for the Disposal of Radioactive Waste (Nagra) carries out experiments and develops models which contribute to safety assessments of disposal concepts for radioactive waste. The staff consists of more than 30 scientists, from a broad range of natural sciences, and technicians. Full experimental demonstrations of the safety case are not possible because of the long timescales associated with the disposal of radioactive waste. The release of radionuclides into the human environment occurs only on time scales of tens of thousands to millions of years after the wastes have been placed in the subterranean repositories. For this reason, safety analyses rely on extrapolations and computations based on a sufficient understanding of the events and processes which influence the long term performance of the repository, and which can affect the transport of radionuclides from the repository back to the human environment. The necessary information is obtained from laboratory experiments and investigations on appropriate natural systems e.g. natural analogues and field experiments in underground research laboratories. On the basis of the results from such investigations, LES develops mathematical models which allow the safety of the disposal system to be numerically assessed.

Geochemical in situ conditions and their temporal evolution: The research in this area involves the definition of the initial system in terms of the groundwater/porewater chemistries, mineralogies in both the near-field and the far-field, radionuclide solubilities, and the modelling of how these may evolve with time, including the interactions between materials such as cement and corrosion products with other near-field materials and the host rock, and how their properties may be influenced through these interactions. “State of the art” specific thermodynamic data bases and speciation codes such as GEMS and reactive transport codes such as MCOTAC are required for this work. These codes are further developed and maintained within LES.

Retention and retardation: The work is predominantly concerned with developing a mechanistic understanding of the uptake processes (adsorption, incorporation, solid solution formation, surface precipitation) of safety relevant radionuclides on relevant systems (argillaceous rocks, bentonite, cement), under realistic chemical conditions and quantifying them in terms of predictive models/codes. This information is then used in the seletion of sorption values for data bases used in performance assessment. The longer term aim is to develop a thermdynamic sorption data base for calculating sorption values under any specified conditions.

Transport in repository systems: The major aim of this work is to understand the diffusion mechanisms in high clay mineral content systems and cement, and develop models to quantitatively describe the processes which can be reliably used to predict the transport of radionuclies over large distances and time scales. The interpretation and modelling of laboratory and field scale experiments and the provision of diffusion parameters belongs to this activity as does also the incorporation of thermodynamic speciation codes (solid solutions) and sorption models into coupled codes in order to develop the capacity to model radionuclide transport more realistically.

Safety Analysis: Knowledge of the near-field (cementitious materials, compacted bentonite, bentonite/ quartz sand mixtures) and far-field systems (argillaceous host rock formations) is put to practical use in performance assessments. Extrapolating the known behaviour of the system allows the release of each of the radionuclides in the disposed waste to be calculated as a function of time. This, in turn, allows estimates of the potential future radiological impact on Man to be made i.e. the effect on health. The combined effect of the predicted individual radiation doses can then be compared with regulatory guidelines, allowing statements to be made about the safety of the repository system.
The design and the demonstration of the safety of such system are tasks which require detailed knowledge and diverse technical expertise and “know how”. In practice this requires the co-operation between many groups and individuals. This is one of the main reason why LES actively seeks integration within the scientific and waste management communities through co-operation and joint projects and fosters connections to universities by providing PhD and Post Doctoral positions within the Laboratory.

Glossary
Radionuclide: Unstable atomic nucleus whose decay follows well established physical laws.
Model: Simplified, frequently mathematical representation of a real system.
Repository Near-Field: The repository structure and its immediate surroundings.
Repository Far-Field: Undisturbed host rock in which the repository is constructed and the surrounding geological strata, reaching up to the surface environment in which humans live.
GEMS: Gibbs Energy Minimisation SelektorMCOTAC:L/ILW: Low and intermediate level radioactive waste
HLW/SF: High level radioactive waste and spent fuelRadiation Dose: Quantity of ionising radiation absorbed by a human.